The present invention relates to a method of acquiring and processing seismic data and in particular to a method of acquiring and processing seismic data that produces spatially filtered seismic data using calculated offset distances.
Seismic data is collected to analyze the subsurface of the Earth, and is particularly collected in connection with hydrocarbon exploration and production activities. Seismic data for analyzing subsurface structures may be collected on land or over water. In order to obtain seismic data, an acoustic source is used which typically consists of explosives or a seismic vibrator on land or an impulse of compressed air at sea. The seismic data acoustic signals reflected by the various geologic layers beneath the surface of the Earth are known as traces and are sensed by a large number, typically hundreds or thousands, of sensors such as geophones on land and hydrophones at sea. The reflected signals are recorded and the results are analyzed to derive an indication of the geology in the subsurface. Such indications may then be used to assess the likelihood and location of potential hydrocarbon deposits.
Seismic surveys are generally conducted using one or more receiver lines having a plurality of receiver station locations spaced evenly along their lengths. In a two dimensional (2D) survey, a single receiver line is used and the acoustic source is typically positioned at various points in-line with the receiver line. In a three dimensional (3D) survey, a plurality of parallel receiver lines are typically used and the acoustic source is generally positioned at various points offset from the receiver lines. While a 2D seismic survey can only create a cross-sectional representation of the subsurface, a 3D seismic survey can be used to develop a three dimensional representation of the subsurface.
Conventional seismic data acquisition systems employ a receiver array at each receiver station location. All of the sensors in a conventional receiver array are connected together (electrically xe2x80x9chardwiredxe2x80x9d) and the receiver array delivers a single output trace at the particular receiver station location about which the sensors are placed. Conventional hardwired receiver arrays perform two important functions when acquiring seismic data.
First, the receiver arrays attenuate ground roll noise. Ground roll is a portion of the acoustic energy produced by the acoustic source that is not transmitted downward toward the subsurface formations, but instead travels horizontally along the earth""s surface. This portion of the seismic signal travels at the Rayleigh wave velocity, which is typically much slower than the velocity of the pressure wave that is transmitted toward the subsurface formations. Although the pressure wave typically travels much faster than the ground roll wave, the pressure wave must travel a much greater distance than the ground roll wave, and the pressure wave and the ground roll wave often arrive simultaneously at a seismic sensor for some portion of the seismic data record. Because the ground roll wave typically contains no information regarding the subsurface geologic structure being investigated, it must be attenuated (i.e. removed) to the greatest extent possible before the seismic data is used to produce maps of the subsurface.
A conventional approach to the problem of ground roll suppression in seismic data processing is to use receiver arrays during data acquisition and then to stack together (i.e. combine/aggregate/add together) the seismic data signals obtained from each of the sensors in the receiver array. Ground roll is generally considered a dominant noise source and effective removal of the ground roll signal often greatly enhances the quality of the subsurface image obtained from the seismic survey. Current seismic data acquisition systems typically employ receiver arrays whose spatial extent is such that noise waves with wavelengths up to 1.4 times the sensor pattern length are attenuated.
A second reason receiver arrays are used in conventional seismic data acquisition systems is to attenuate random noise. By making numerous measurements of the seismic response at a particular receiver group location using different sensors, random noise can be attenuated by combining the readings from the different sensors. Often more than twenty sensors are used to make up the receiver array at a particular receiver station location. For a group of twenty-four geophones at a receiver station, the signal to noise ratio of the stacked output trace will typically be increased by fourteen dB compared to the readings of each of the individual sensors. If only five geophones were used at the receiver station, the signal to noise ratio will typically be increased only by about seven dB compared to the readings of each of the individual sensors.
The use of conventional hardwired receiver arrays has some distinct disadvantages, however, both from a geophysical point of view and from an economic point of view. Their use leads to a spatial smearing effect: the response at a particular receiver station location is the sum of the spaced apart individual sensors in the receiver array at that location. There is also a trend in the industry towards smaller bin sizes. The standard 50xc3x9750 meter bin sizes will likely be reduced in many instances to 40xc3x9740 meters or 30xc3x9730 meters, for instance, to overcome spatial aliasing problems and to increase resolution. As an example, high resolution is required for reservoir monitoring to establish 3D-impedance maps of the reservoir. This concept of smaller bin sizes is compromised by the spatial smearing effect introduced by conventional receiver arrays. The use of smaller sensor pattern lengths also limits the wavelengths of the ground roll that can be attenuated by stacking together the seismic responses received by each of the sensors.
The costs associated with conventional hardwired receiver arrays are also problematic. Conventional hardwired receiver arrays are expensive to manufacture and maintain due to the large number of sensors needed at each receiver station and the lengths of the electrical cable needed to allow the sensors to be spread out at the receiver station location to form the receiver array. Even more significant is the cost of utilizing these systems in the field, particularly the cost of deploying the sensors in the extremely difficult to access areas in which seismic surveys are often conducted, such as swamps, jungles and forests.
It is an object of the present invention to provide an improved method of acquiring and processing seismic data that eliminates many of the disadvantages of conventional seismic data acquisition systems that utilize hardwired receiver arrays to attenuate ground roll and random noise.
An advantage of the present invention is that the separation of the reflection energy and the ground roll energy in the F-K domain can be maintained for large cross line offsets in 3D seismic surveys.
Another advantage of the present invention is that it can be used to effectively filter seismic data obtained using a variety of seismic sensor layouts, including crooked receiver lines and non-uniformly spaced receiver station locations.
A further advantage of the present invention is that ground roll and random noise can be effectively attenuated as the seismic data is spatially resampled or is prepared for resampling.
The present invention can also be used effectively with sensor layouts having smaller cross-line areal footprints and fewer sensors per receiver station location than conventional seismic data acquisition systems.
According to the present invention there is provided a method of acquiring and processing seismic data, the method comprising the steps of deploying a plurality of seismic sensors, actuating a seismic source, receiving seismic signals produced by the seismic source using the seismic sensors, calculating offset distances between the seismic source and the seismic sensors, and producing spatially filtered seismic data using the received seismic signals and the calculated offset distances.
In a preferred embodiment, the seismic data is spatially filtered using a frequency independent wavenumber filter and the range of passband wavenumbers used is directly related to the anti-aliased spatial resampling of the seismic data. The use of such a method can produce ground roll and random noise attenuated output traces that are virtually free of spatial aliasing.
In this embodiment, the range of wavenumbers in the seismic data is restricted by spatially convolving the seismic data in the time domain with a wavenumber filter of the form:       f    ⁢          (      q      )        =      2    ⁢          k      0        ⁢                  sin        ⁢                  (                      2            ⁢            π            ⁢                          xe2x80x83                        ⁢                          k              0                        ⁢            q                    )                            2        ⁢        π        ⁢                  xe2x80x83                ⁢                  k          0                ⁢        q            
where the range of wavenumbers in the seismic data is restricted to xe2x88x92k0xe2x89xa6kxe2x89xa6k0, and q is the reduced signed offset sensor coordinate.
The filter used may be truncated by limiting the range of values of 2k0q to 2 or 3 for instance, to avoid excessive filter lengths. A filter that gives the most optimal results for the given filter length can then be determined.
Also in this embodiment, the wavenumber spectrum of the seismic data is restricted during the data acquisition phase of the seismic survey and the residual portion of the ground roll energy in the seismic data may be removed by frequency dependent wavenumber filtering during later data processing.
Further preferred features of the present invention are set out in the dependent claims.